1. Field of the Invention
The present invention relates to a diesel engine in which the premixed combustion and normal combustion are switched based on the engine operation state.
2. Description of the Related Art
In diesel engines, fuel is typically injected close to a compression top dead center of the piston when the temperature and pressure inside the cylinder are high. In this case, the fuel is ignited producing a flame during fuel injection, and the combustion is maintained by supplying subsequently injected fuel into this flame. In such conventional combustion mode, there is a portion of the initially injected fuel that is combusted after the ignition delay period and the portion combusted in the combustion gas with and insufficient amount of oxygen, thereby raising a problem of NOx and smoke generation. In the present specification, the combustion mode in which the fuel ignites during the injection is called “normal combustion”.
Accordingly, the applicants have suggested a diesel engine in which the fuel injection timing was before the compression top dead center and the mixture was ignited after the end of fuel injection (Japanese Patent Application Laid-open No. 2003-83119).
In such a diesel engine, the mixture is ignited once a certain period elapses after the end of fuel injection. As a result, the mixture is sufficiently leaned and homogenized before the ignition. Therefore, local combustion temperature decreases and the amount of released NOx (nitrogen oxide) is decreased. Furthermore, the emission of smoke is also inhibited because local combustion in an air-deficient state is avoided. In the present specification, the combustion mode in which the mixture is ignited after the end of fuel injection will be called “premixed combustion” and the period from the end of fuel injection to the mixture ignition will be called “premixing period”.
The premixed combustion is effective for improving the exhaust gas, but if the premixing period is insufficient, the effect of improving the exhaust gas cannot be obtained or fuel consumption is sometimes degraded. For example, if the premixing period is too short, leaning and homogenization of the mixture are insufficient and the effect of exhaust gas improvement is decreased. Furthermore, if the premixing period is too short and the ignition timing of the mixture is before the top dead center, the fuel consumption is degraded because the compression created by the piston is received after the ignition.
The applicants repeated various trial and error to resolve the above-described problem. The results obtained demonstrated that a sufficient premixing period can be ensured by implementing the EGR (exhaust gas recirculation) of a comparatively large quantity (EGR ratio of the mixture is increased) in combination with early injection of the fuel. In other words, if the EGR ratio is increased, the concentration of oxygen in the mixture decreases. Therefore, the premixing period can be extended. This technology was not publicly known at the time the present application was filed and does not constitute the prior art. In the present specification, the premixed combustion accompanied by the EGR of a comparatively large quantity is called “PCI (Premixed Compression Ignition) combustion”
However, the PCI combustion is difficult to realize in a high-load region of the engine. The reasons therefor are explained below.
(1) In the high-load region of the engine, the fuel injection quantity increases. Therefore, a large quantity of oxygen (air) is necessary to ensure the adequate air/fuel ration. Thus, the EGR of even larger quantity has to be conducted to ensure a sufficient premixing period in the state in which the quantity of fuel and oxygen increases. In other words, a large quantity of air (new air) and a large quantity of EGR gas (exhaust gas) have to be introduced into the combustion chamber. However, an intake system (supercharge system) capable of realizing such a process has not yet been developed.
(2) In the normal combustion, the fuel that is being injected burns by small portions (portion after portion). By contrast, in the PCI combustion, the entire injected fuel burns after it is mixed. Therefore, the maximum pressure in the cylinder is higher than that of the normal combustion. In a high-load region where the fuel injection quantity increases, the maximum pressure inside the cylinder further increases. Therefore, there is a possibility of the engine strength being insufficient.
Accordingly, the applicants suggested to switch the PCI combustion and normal combustion according to the engine operation state (in particular, engine load). In other words, the PCI combustion is implemented in the region with a comparatively low engine load, and the normal combustion is implemented in the region with a comparatively high engine load.
Here, there is a comparatively large difference in the required (adequate) EGR ratio between the PCI combustion and normal combustion. This difference can be explained as follows. In the PCI combustion, as described above, the premixing period is extended by the EGR of a large quantity and the exhaust gas is improved. However, in the normal combustion, if the large-quantity EGR is implemented, then a local oxygen insufficiency occurs and smoke is generated. Therefore, the EGR ratio has to be suppressed to a certain low level.
Therefore, when the engine operation state moves from the PCI combustion implementation region to the normal combustion implementation region or in the opposite direction, the EGR has to be changed rather significantly.
However, because the EGR gas and intake air are compressible, the EGR ratio of the mixture is difficult to change rapidly. In particular, in the external EGR apparatus, which returns part of the exhaust gas present in the exhaust channel into the intake channel, the delay of the changes in the EGR ratio becomes more significant due to the response delay of the EGR valve and the existence of the volume portion associated with the distance (intake path) from the EGR valve to the combustion chamber.
By contrast, the fuel injection timing can be controlled with a comparatively high accuracy by controlling the electric actuation timing of the injector. Therefore, switching between the PCI combustion and normal combustion can be conducted rapidly. As a result, a mismatch occurs between the fuel injection timing and EGR ratio immediately after switching between the PCI combustion and normal combustion. This can cause degradation of exhaust gas or fuel consumption.